In a conventional copying machine or a microfilm reader/printer, light beams from an original pass through a projection optical system and are sequentially scanned using a scanning mirror, thereby projecting and focusing an image on a rotating photosensitive drum.
A conventional image scanning apparatus which guides light beams passing through a projection optical system onto a photosensitive drum surface is disclosed in U.S. Pat. No. 4,299,480 and Japanese Patent Disclosure (Kokai) No. 55-126258.
In this conventional apparatus, scanning operation is performed by moving the scanning mirror while changing the inclination of the mirror, so that a distance from a reflection point of the light beams on the scanning mirror surface to an imaginary focal plane is equal to a distance to a reference point on the photosensitive drum. In this scanning method, good optical performance of a projected image formed on the photosensitive drum can be maintained, but the scanning orbit of the scanning mirror becomes nonlinear, resulting in a very complicated scanning mechanism.
More specifically, in order to perform optical scanning in the conventional apparatus, as shown in FIG. 1, a single scanning mirror 106 is disposed at a substantially intermediate position between a focusing lens 104 and an imaginary focal plane 105. Since the scanning mirror 106 must move in synchronism with the rotation speed of a photosensitive drum 107 while the reflection surface of the scanning mirror 106 always coincides with perpendicular bisectors of line segments connecting respective points on the imaginary focal plane 105 and a focal point D on the photosensitive drum 107, the orbit of the scanning mirror corresponds to curve SS'.
However, in the conventional apparatus, the image scanning apparatus for moving the scanning mirror is arranged so that the scanning mirror is moved by a slidable link member while linking with a pupil position of the lens or a portion corresponding to the exposure position on the photosensitive drum, the apparatus becomes bulky, and the margin of an actual optical arrangement is decreased.
Another method is proposed wherein the scanning mirror is rotated to perform scanning without moving it. With this method, an optical path difference is generated, and a reproduced image on a photosensitive drum surface may cause a barrel-like or trapezoidal distortion.